The term "light guide" is used to designate a homogenous bar of refractive material (e.g. glass or plastic material) which is elongate and generally cylindrical in shape. One of the ends of the bar the light guide receives light produced by an adjacent light source (this end is referred to as the "adaptor" or "coupling adaptor"), and light rays received by the adaptor are transmitted along the guide by multiple reflections at the interface between the refractive medium and air.
It can be shown, that for a cylindrical light guide, any ray of light which has been subjected to a first reflection on the inside of the cylindrical surface will necessarily strike said surface on subsequent occasions at an angle of incidence which is always greater than the minimum angle required for reflection. The ray will therefore continue to be reflected along the guide and is thus "trapped" therein until it reaches the other end, herein referred to as the "outlet end".
This condition remains substantially true even when the rod is slightly curved rather than being geometrically straight, thereby enabling light rays to be guided along a curvilinear path. However, the following description will be limited to the case of a straight cylindrical light guide (which in any case is true in the immediate vicinity of the coupling adaptor) even though the invention is equally applicable to curvilinear guides provided their radius of curvature remains large relative to their diameter.
The aim generally sought when designing a coupling adaptor is to capture a maximum light flux at the inlet to the guide. The property of "trapping" light rays only applies to rays which have entered the refracted medium and which have been subjected to a first reflection on the inside of the cylindrical surface of the guide; all other light rays emitted by the source are pure loss.
For a light guide, any ray striking the surface of the adaptor penetrates into the guide, and it may additionally be shown that if the surface of the adaptor is plane or convex, said ray will always be reflected by the inside cylindrical surface of the guide. Thus, in this case, any ray striking the surface of the adaptor is necessarily "trapped" inside the guide, regardless of its initial angle of incidence.
The light flux captured by the guide relative to the total flux emitted by the source is then determined by the solid angle centered on the source and intercepting the area of the adaptor (where the source is assumed to be a point source or a source which has been made equivalent to a point source).
In order to capture a maximum amount of light, it is therefore necessary either to bring the source close to the adaptor, or else to increase the area of the adaptor, with a consequent need to increase the diameter of the guide. The nearest the adaptor can be brought to the light source is determined by the size of the globe in which the light source is contained. The largest usable inlet area is limited by problems associated with excessive bar size, difficulties in curving the guide, and the quantity of material required to make the guide.
It is tempting to increase the amount of light flux which is captured by giving the adaptor surface a shape which is neither plane nor convex, but which is concave, thereby increasing the value of the above-specified solid angle and consequently increasing the proportion of light rays which penetrate into the guide.
However, when the surface of the adaptor is concave, it can be shown that some of the light rays emitted by the source and penetrating into the refractive medium subsequently pass through the cylindrical surface of the light guide since their angle of incidence on said surface is less than the limiting angle for total internal reflection in the refractive medium. These rays which leave the guide early are thus not collected, and as a result the light flux effectively conveyed by the light guide is generally less than it would have been if the adaptor surface were plane or convex.
It can thus be seen that the solution of hollowing out the adaptor to have a concave surface in which the light source is received is not, of itself, adequate to increase the light flux effectively conveyed by the light guide. Worse, it is observed that the quantity of light flux which is effectively conveyed by the light guide is less than the quantity which would have been conveyed using a plane or convex adaptor.
Preferred embodiments of the present invention provide a coupling adaptor for a light guide giving increased light flux capture.